I have installed on my machine the Adobe CC suite, I need to get access to Adobe CS4 in order to work on a legacy project that uses AS2 and eventually upgrade to CS6 what are the steps that I'm supposed to take in order to achieve this ?
In addition to what ClayUUID said, Flash Professional CS4 is no longer available for purchase. If you have a previous version with you, you can install it alongside Flash Pro CC.
If you are subscribed to Flash Pro CC, use the link provided previously to install Flash Pro CS6.
And in addition to what the others said, CS6 can still make AS2 SWFs, it should be able to open your old FLA, no need to go back to CS4.